The most common orthopedic condition for which professional medical treatment is sought is lower back pain. Although many factors may be responsible for causing lower back pain, a principal factor is damage or degeneration of an intervertebral spinal disc resulting in impingement on the nerve system, specifically the spinal cord, located within the spine. Such impingement may result in, for instance, loss of mobility, urinary and fecal incontinence, and sciatica or pain experienced in the extremities.
Damage to or degeneration of a spinal disc can result from a number of factors such as abuse or age. The disc itself is composed primarily of an annulus and a nucleus contained therein. The annulus is a fibrous annular piece that attaches to the adjacent vertebrae and contains the nucleus, which is in turn a gel-like viscous material capable of shock absorption and flowable to permit poly-axial rotation and resilient compression of the vertebrae and spine. Most frequently, disc degeneration results from damage occurring to the annulus such that the flowable nucleus material may leak or seep out of the annulus. Disc degeneration also can occur in other ways, such as by being deprived of nutrient flow leading to a dried and susceptible to damage disc. Because the nuclear material is flowable, extensive damage to the annulus is not necessary for leakage to occur.
A recent, though not new, development for spinal surgery is a procedure known as disc arthroplasty for restoring or reconstructing the disc using a prosthesis to replace a portion or entirety of the damaged disc. The primary objective of disc arthroplasty is to restore or maintain the normal disc anatomy and functions, while addressing and treating the causes of the pain.
Two types of prostheses for disc arthroplasty are currently believed to merit further development by medical science and research. One type is a total disc prosthesis, or TDP, where the entire spinal disc is replaced after radial discectomy. A typical TDP includes structures that together mimic the properties of a natural disc.
The other type is a disc nucleus prosthesis, or DNP, that is used to replace only the nucleus of a spinal disc after a nucleotomy while retaining the annulus of the disc and, possibly, the end plates intact. As discussed above, failure of the natural disc does not require extensive damage to the annulus, and the annulus would often be capable of retaining a non-flowing prosthetic nucleus. Implantation of a DNP involves clearing of the natural nucleus from the annulus through the procedure known as nucleotomy, and inserting the DNP within the annulus. Accordingly, disc nuclear prostheses (DNPs) are typically smaller and require less extensive surgery than TDPs do.
An issue related to DNPs is implant extrusion, defined as the tendencies for an implant not to remain seated, and for the implant to back out of its intended seat in the nuclear space. To prevent this, many designs for disc implants attempt to secure to the end plates of the vertebrae by providing securement features on the implant. The nuclear implants may have one or more restraining features, such as, for example, keels or other implant protrusions that seat into the bone, apertures integrated into the implant for bone in-growth such as a porous surface or coatings, or screws to screw the implant to the bone. These and other similar features restrain the implant in a predetermined orientation to the surrounding boney bodies to thereby properly support the skeletal structure and prevent damage of any soft tissues. These features, however, may violate the integrity of the end plates to a degree where revision surgery is limited. Violation of the vertebrae by the securement may cause bleeding, or calcification of the end plate, either of which can result in pain, loss of mobility, necrosis, or deterioration of any implant device.
Some arthroplasty devices are designed to float or sit unrestrained within a ligamentous joint capsule. These devices may rely purely on the soft tissue holding the replacement device in the predetermined position. An unrestrained intervertebral artificial nucleus device would benefit from an intact annulus to secure the implant in the predetermined position and prevent its expulsion into the sensitive nerve structure located just outside the annulus. The health of the annulus, however, is often compromised through the degenerative disc disease process and may not be intact. The annulus may have tears or may be poorly nourished and weak such that it cannot adequately serve by itself to restrain the nucleus replacement device within the confines of the annulus. Additionally, the annulus is typically at least partially incised during surgery to make an opening for removal of the diseased nucleus material and to serve as a window for placing the nucleus replacement device in its predetermined position. It is possible for this window to serve as an undesired expulsion portal for the nucleus implant.
For these and other reasons, the implant retention devices described herein may be utilized to assist in the retention of a nuclear implant, particularly those that do not have other restraining features, in a predetermined skeletal relationship.